An image scanner may include a linear array of image sensors (a scan bar) configured to scan an image line by line. For example, a scan bar may include a row of image sensors configured to simultaneously scan the width (X axis) of a document. The scan bar may then move relative to the document in order to incrementally scan the entire length (Y axis) of the document line by line. The resolution in the X axis may be limited by the density of the sensors on the scan bar and the resolution of the Y axis may be limited by the distance a stepper motor advances the scan bar with respect to the document with each incremental step.
For example, in a single-pass multi-channel image scanner, a scan bar configured to scan an 8.5-inch-wide document at a maximum resolution of 300 DPI may require 2,550 sensors in each of the Red, Green and Blue channels, for a total of 7,650 sensors across the width of the scan bar. The same image scanner configured to scan the length of a document with a maximum resolution of 600 scan-lines per inch may require a stepper motor capable of advancing the scan bar in increments of 1/600th of an inch. Alternatively, a scan bar may be advanced at a constant known speed across the length of the document as each scan-line is incrementally recorded in a memory buffer. In the case of an automatic document feeder (ADF), a document may be advanced at a constant speed past a scan bar.
The exposure of each scan-line of a document is based on the sensitivity of sensors, the intensity of a light source, and the amount of time each scan-line is illuminated by the light source. To achieve a proper exposure (not too light and not too dark), each scan-line must be advanced past a constant light source for a specific amount of time. If a document is fed too fast past a scan bar having a constant light source and sensor sensitivity, the image may be underexposed. Similarly, if a document is fed too slowly past a scan bar having a constant light source and sensor sensitivity, the image may be overexposed.
The exposure systems of traditional image scanners, such as those using xenon or fluorescent light sources, require that a document be scanned at a pre-determined scanning rate in order to ensure proper exposure. In such systems, a constant motor speed (in the case of a flatbed scanner) or a constant document speed (in the case of an ADF) is required in order to ensure a proper and even exposure of the entire document during the image scan.
Problems arise when a memory buffer or the image processor fails to keep up with pre-established scanning speeds. In a flatbed scanner, it may be possible to stop the scan bar when the memory buffer becomes full, and then restart the scan bar when the memory buffer catches up (referred to as a start-stop cycle). Each start-stop cycle may involve retracting the scan bar in order to allow for mechanical acceleration of the scan bar as it approaches the location where the scan last stopped (referred to herein as a “retract-and-accelerate” approach). If the scan bar is not retracted, scan-lines recorded by the scan bar during the mechanical acceleration of the scan bar may be overexposed. The motor driving the document (through an ADF) or the scan bar (on a flatbed scanner) must be moving at a constant and known velocity in order to ensure proper exposure. Each start-stop cycle may involve complex mechanical movements in order to perfectly align the resulting scanned portions.
Traditional attempts to achieve accurate start-stop cycles on a flatbed scanner have included using accurate stepper motors or constant drive motors configured to provide accurate retraction and dampening of the scan bar. However, the mechanical systems of ADFs often include complex gearing, clutches, and other components that make the retract-and-accelerate approach difficult or cumbersome. In many ADF systems, the required mechanical precision does not exist for start-stop cycles involving regression. Accordingly, some approaches simply advance a document in an ADF at a constant speed for the entire duration of the scan. Such an approach requires a slower ADF, larger buffers, faster downstream image processing, and/or reduced downstream image processing.
In the following description, numerous specific details are provided for a thorough understanding of the various embodiments disclosed herein. The systems and methods disclosed herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In addition, in some cases, well-known structures, materials, or operations may not be shown or described in detail in order to avoid obscuring aspects of the disclosure. Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more alternative embodiments.